The Attachable Grinder allows you to put a miniature grinder on 6 different mobs: Pigs, Cows, Chickens, Creepers, Zombies, and Skeletons. It gives you the ability to harvest drops from these mobs without actually killing them… initially. After a set number of items drop from the mob it will explode. This can all be changed in the config file though.
If you set the yield to be infinite without exploding in the config then this is a fantastic way to get resources in a seemingly unlimited way as they will then continually drop items. You could set up a pen that houses just one of these mobs and it will constantly fill up chests you have it linked to.